This is a topic that seems to come up more and more often, and one that I've given a great deal of thought to.
The assumption is that treatment free beekeepers are maintaining 'mite bombs' that negatively impact the 'responsible' beekeepers by robbing/drift....act as a reservoir for mites that re-infest the hives of those who have recently and responsibly treated their bees.
....when I hear people make these claims, the first thing that comes to mind is 'bee trees'. I don't know a beekeeper that doesn't have a warm spot in their heart for a bee tree...most of the time (unless there is a nuisance issue or the tree is coming down for another reason), beekeepers are happy to leave them be (and maybe setup a swarm trap or 3). ...but why is this? Why is the bee tree not just another (eliminateable) mite bomb?
...more to the point (and I'll use conservative numbers here), there are about 2.5 million migratory bee colonies in the US. Let's assume an _average_ (some will be much less, some will be much more) escaped swarm rate of 1% migratory colony/year.
That 'deposits' 25,000 unmanaged colonies into our environment per year. ...but unlike the 'bombs' being maintained by beekeepers using a TF approach, these swarms are from stock that is idealized for migratory beekeeping (mites are presumed to be kept down by treatments while early/ready brooding for almonds and/or package production are among the primary selected traits).
Certainly not all hives being maintained TF are from mite resistant stock, but there is certainly a strong bias in that direction.
So, there are 1 or 2 thousand migratory commercial beekeepers in business that are responsible for 25,000 'mite bombs' that are more suited for the role of 'mite bomb' than most TF colonies...they've even been exposed to all the emerging pathogens at the annual pox party before being shipped around the countryside casting off swarms.
If 'mite bombs' were really a concern, we would be talking about these aprox 25,000 swarms/year.
Whereas a TF beekeeper is generally 'loosing' when their hive collapses, the 'cost' of these swarms is already (by the fact that more rigorous swarm prevention isn't common practice) been determined to be just a cost of doing (profitable) business.
I'm not out to criticize anyone or any particular approach....my criticism is with the lack of 'big picture thinking' that goes on when TF beekeepers are accused of being the problem.